Steps to reduce the apk size.

Reduce_apk_size.txt

1- reduce the other's language strings in string.xml
add below in build.gradle-

 defaultConfig {
        resConfigs "en"
    }
    
2- unzip the final apk, go to res directory, run the below commamd to know the size of resfiles in descending order.
find . -type f -exec ls -al {} \; | sort -nr -k5 | head -n 25

3- 
EXAMPLE:

removed okhttp -> 4,072,187 bytes  4.1 mb
removed karim  -> 4,025,299 bytes
  Original number of program classes: 4450


removed alfodan Dialogs  -> 3,668,810 bytes (3.7 MB on disk)
Original number of program classes: 4040

Removed Flurry -> 3,567,670 bytes (3.6 MB on disk)

3,626,086
3,685,543 bytes enable karim 
3,670,960
3,452,772 bytes  -> removing intransitive dependencies 

3,457,166 bytes -> enable Circular
3,463,558 bytes -> enable DND , used http url connection
total class :> 3678

3,463,558 bytes -> enable DND , used http url connection
total class :> 3678

3,466,428 bytes (3.5 MB on disk) -> Used the Appcompact Dialogs 
total class :> 3681

3,611,034 bytes (3.5 MB on disk)  -> Added Crashlytics ~145 KB 
total class :> 4044

3,301,799 bytes (3.3 MB on disk) ->  only en-resource build.xml 

3,258,534 bytes (3.3 MB on disk) -> removed some images 

3,229,536 bytes (3.2 MB on disk) -> removed coming soon image

3,258,363 bytes (3.3 MB on disk)

design support material tab up to 14+

Current app 3.65 MB download.
New app 2.77 MB download.